Frictional materials such as a disc brake pad and a brake lining are used for braking a car and the like. These frictional materials play a role in the brake by frictioning the counter members such as a disc rotor and a brake drum. Therefore, for the frictional materials, not only high friction coefficient and stability of this friction coefficient are required, but also a prolonged pad life in the use of the brake in a wide temperature range from low to high temperature, that is, the abrasion resistance, is required.
When the brake is used in the high temperature range, a lump formed of metal abrasion powders, which is called a metal catch, is generated on surface of the frictional material, whereby sometimes causing a squeal of the brake as well as leading to increase in abrasion loss of the disc rotor and the frictional material. Accordingly, a proposal was made to blend a metal sulfide to suppress generation of the metal catch as well as to improve the abrasion resistance at high temperature (see the Patent Document 1).
On the other hand, a frictional material contains a binder, a fiber substrate, an inorganic filler, an organic filler, and so forth; and in order to express the above-mentioned characteristics, in general, these are respectively used solely or as a combination of two or more kinds of them. As to the fiber substrate, an organic fiber, a metal fiber, an inorganic fiber, and the like are used; and to improve the abrasion resistance, a copper fiber and a copper alloy fiber are used as the metal fiber. Furthermore, in the frictional material, a non-asbestos frictional material has become a main stream, in which a large amount of a copper, a copper alloy, and the like are used.
However, in the frictional material containing a copper and a copper alloy, the abrasion powders generated during braking contain a copper, suggesting possibility of causing pollution in a river, a lake, an ocean, and so forth; and therefore, a movement to control the use thereof is growing. Accordingly, a method is proposed wherein the frictional material not containing metals such as a copper and a copper alloy while containing a magnesium oxide and a graphite with the content thereof being 45 to 80% by volume and with the ratio of the magnesium oxide to the graphite being in the range of 1/1 to 4/1 is used (see the Patent Document 2).    Patent Document 1: Japanese Patent Laid-Open Publication No. 2003-313312    Patent Document 2: Japanese Patent Laid-Open Publication No. 2002-138273